Hot Water Cylinder Thermostat

karlitob

Hi all

Your advice and guidance would be obliged.

For the last two days, the hot water has been lukewarm.

Leaks:
- no leaks
- overflow pipe not overflowing
- I went up to the attic and no refill issues ie the cold storage tanks is not continually refilling.


Motorised Head Valve
- no problems
- it’s turns on and off as I turn up and down the thermostat


Immersion
- no problems
- when I use the immersion, the water heats up correctly.


Boiler and Cylinder Thermostat (Horstmann HCT2)
- both set to 65C as normal.
- I’ve checked all electrical connections on the cylinder thermostat.

So no leaks and the motorised head valve works - the problem I’ve found was after an hour of heating the water (which normally heats the cylinder it fully), the thermostat read only 40C. I discovered this while testing the head valve - I had to turn it all the way down to 40 before the head valve turned off. So the thermostat is calling for heat but it’s not heating higher than 40C.



Is it reasonable to assume that it’s the thermostat?


Thanks all.

meercat

Most likely alright
Can you unclip the thermostat from the cylinder for a day or two to see does water heat up fully

karlitob

meercat wrote: »

Most likely alright
Can you unclip the thermostat from the cylinder for a day or two to see does water heat up fully

Good idea - hadn’t thought of that. Thank you.

John.G

If your (electric) immersion heats the whole cylinder then you can assume its heating it to 60C (if left on long enough) then by turning the cylinder stat up/down you should see if it corresponds to ~ 60C, proving its OK.

Check your boiler stat set ~ 65C or greater.

Check also if you have a balancing valve on the outlet from the cylinder coil, this is normally a gate (wheel head) valve, if fitted, just open and close it fully a few times ~ 3 to 4 full turns each way, then close it fully and finally open it ~ one full turn or so.

Also could be air trapped in the cylinder coil, might have to slacken coil flow at connection (top) and release any air if no air vent fitted, either manual or automatic (AAV).

karlitob

John.G wrote: »

If your (electric) immersion heats the whole cylinder then you can assume its heating it to 60C (if left on long enough) then by turning the cylinder stat up/down you should see if it corresponds to ~ 60C.

Check your boiler stat set ~ 65C or greater.

Check also if you have a balancing valve on the outlet from the cylinder coil, this is normally a gate (wheel head) valve, if fitted, just open and close it fully a few times ~ 3 to 4 full turns each way, then close it fully and finally open it ~ one full turn or so.

Also could be air trapped in the cylinder coil, might have to slacken coil flow at connection (top) and release any air if no air vent fitted, either manual or automatic (AAV).

Good ideas. Thank you.

Didn’t want to mess with balancing valve.

Boiler at 65C

Automatic air vent also fitted.

Will try the immersion/stat idea. Actually had thought of that and forgot to do it last night.

My immersion element itself doesn’t have an obvious thermostat so I presume it’s built in and under the cap at the top. But I understand that you mean to hear the water by the immersion, then turn on the gas to see if the thermostat is corresponding to a higher temperature.


Thank you.

John.G

You won't really mess up the balancing valve, just firstly count how many turns (or part of a turn that it takes to close it, then do what I suggested.

The AAV should have a little cap on the air release nozzle which is often left shut, just slacken this a few turns with any isolating valve open, if no air then you should still get a few drops of water, these AAVs, if not fitted recently can also have a blocked nozzle which gives the impression of no air problems. Also feel the cylinder coil flow & return, the flow should be very hot and the return, not quite as hot but still very warm/"hot"

Yes, the electric immersion has its own stat.

karlitob

John.G wrote: »

You won't really mess up the balancing valve, just firstly count how many turns (or part of a turn that it takes to close it, then do what I suggested.

The AAV should have a little cap on the air release nozzle which is often left shut, just slacken this a few turns with any isolating valve open, if no air then you should still get a few drops of water, these AAVs, if not fitted recently can also have a blocked nozzle which gives the impression of no air problems. Also feel the cylinder coil flow & return, the flow should be very hot and the return, not quite as hot but still very warm/"hot"

Yes, the electric immersion has its own stat.

Thank you for all that.

Will review the AAR and review the valve.

Flow is very hot but I didn’t check the return - will do that also.

Thanks again.

karlitob

karlitob wrote: »

Thank you for all that.

Will review the AAR and review the valve.

Flow is very hot but I didn’t check the return - will do that also.

Thanks again.

So

- checked balancing valve as suggested.
- checked AAR, no issues
- took off the thermostat

- no real change at all. What does this mean?

As before I’ve noticed that the ‘clicking’ of the thermostat (the switch as such) happens near the lowest setting (the frost symbol.


So is it still a good idea to buy a new thermostat - the low temperature the thermostat is switching offf the motorised head valve is happening regardless whether the thermostat is on or off the cylinder. But the immersion works fine. And there’s no leaks.

Thermostat is the culprit????

John.G

Re thermostat: If you have the stat removed from the cylinder then it will be sensing the room/hot press temperature of say ~ 15/20C, just assume 15C, then turn up the setting knob up fully to ensure that the stat is calling for heat, then when you turn it down you shouldn't hear it clicking OFF until the it reaches ~15C, if you then slowly turn it up again it should click in again at ~ 25C which will prove that (1) it is measuring the temperature correctly, and (2) that it is switching correctly which is ON @ setpoint (& actual temp) -10C and OFF at the setpoint (& actual temp)
More simply, let it removed from the cylinder and turn it up to max and see if the zone valve is opened and stays open and that the boiler is actually firing up and stays firing, if your cylinder still doesn't reach 50/60C in say 45 minutes then your problem is elsewhere and is not the stat, you said in first post that when attached to the cylinder that it was switching on/off around the 40c cylinder temperature which suggests to me that there is nothing wrong with the stat. What was it doing when you heated up the cylinder via the immersion? was it switching out at ~ the cylinder temperature and then switching back in when increased by ~ 10/12C?.

How far open is the balancing valve now? and how far open do you reckon it was opened originally? and is the coil return "hot" as well as a very hot coil inlet?.

John.G

karlitob wrote: »

So

- checked balancing valve as suggested.
- checked AAR, no issues
- took off the thermostat

- no real change at all. What does this mean?

As before I’ve noticed that the ‘clicking’ of the thermostat (the switch as such) happens near the lowest setting (the frost symbol.


So is it still a good idea to buy a new thermostat - the low temperature the thermostat is switching offf the motorised head valve is happening regardless whether the thermostat is on or off the cylinder. But the immersion works fine. And there’s no leaks.

Thermostat is the culprit????

Just to be sure to be sure be happy that the boiler IS actually firing up when the HW zone valve opens as it is actually a switch in the zone valve actuator that enables the boiler

karlitob

John.G wrote: »

Just to be sure to be sure be happy that the boiler IS actually firing up when the HW zone valve opens as it is actually a switch in the zone valve actuator that enables the boiler

Balancing Valve
- was about 2/3 open and now 1/3 open, if that makes sense.
- takes 12 quarter turns to open (my measurement) from closed. It was 8 turns open from closed and now 4 turns open from closed.

Thermostat.
- thermostat off the cylinder, set to maximum and boiler set to 65C
- motorised head valve opens and calls for heat, boiler definitely working. Boiler reaches 65C and turns off when hits 65C but stays on.
- when I start turning the thermostat down, it ‘clicks’ at ‘frost’ symbol, and the head valve turns off boiler. The valve opens and turns back on boiler when I turn the stat back up. There is no other clicking sound.
- the copper cylinder itself in the space behind the stat is not that hot.

- I then put the stat back on the cylinder. All the above is the same except the ‘click’ is at 40C

- I then turned over to the immersion. And let that run for a good while. And it works - our ‘normal’ hot water. However, the stat is still clicking at 40C - very confusing.


So I’m a bit stumped.

- I do think the thermostat is broken but it’s not so clear as to what the main issue is. The stat is calling for heat, the valve is open, the boiler is hot and the flow and return are roasting! There’s no air circulation issues.

- the only thing I think it to try to close the balancing valve a little more. And to buy a thermostat and see what happens cos they are cheap.

- one final point is that I recently had problems with our downstairs radiator motorised head valve. It’s all fixed now - I change the actuator. However - for a few weeks over Christmas I had the actuator off the valve so whenever the hot water was on it also turned on the downstairs radiator as there was no closed valve. The cylinder coil and the thermostat likely never worked so hard before - any thoughts on whether that is related.


Thanks again to everyone for their help and advice.

John.G

OK then, The balancing valve opening of 1 full turn open (4x1/4) should be fine, if you do throttle it in further I would suggest no more than by 1/2 of full turn (2X1/4), I keep mine full open as I have no problem with balancing my rads. There is a theory that by throttling it in that it gives more time for the water to give up its heat but IMO false as the faster the water circulates through the coil, the more turbulence which gives better heat transfer, also because the mean coil temperature is higher which means that the difference between the coil temperature and the cylinder water is greater which also leads to greater heat transfer and faster warm up.

Yes, you may as well renew the cylinder stat. see list of queries, below.

I don't think there is any relationship between your downstairs rad and present problem.

Queries:

Have you a gas or oil fired boiler, if gas then especially when the cylinder water is cool, it should continue to run (after modulating down) continuously until the cylinder water reaches ~ 50/55C, then it may start cycling on/off until final temperature reached. If oil fired it will cycle on/off continuously so hard to measure except to say the the OFF cycle times will increase as the cylinder temperature rises, it may stay off for 2 or 3 minutes initially and off for 5 to 7 minutes when cylinder getting up to temperature.

Also see where the cylinder stat is located (strapped) to the cylinder, it should be above (maybe by 6" or so) the coil outlet, if below it (unlikely) it won't measure the proper temperature.

Do you live in a hard water area, if so, this will affect the coil transfer and both flow and return will be "roasting", but it takes time for the limescale to build up and if your heating was OK a few days ago then it certainly isn't your problem now.

karlitob

John.G wrote: »

OK then, The balancing valve opening of 1 full turn open (4x1/4) should be fine, if you do throttle it in further I would suggest no more than by 1/2 of full turn (2X1/4), I keep mine full open as I have no problem with balancing my rads. There is a theory that by throttling it in that it gives more time for the water to give up its heat but IMO false as the faster the water circulates through the coil, the more turbulence which gives better heat transfer, also because the mean coil temperature is higher which means that the difference between the coil temperature and the cylinder water is greater which also leads to greater heat transfer and faster warm up.

Yes, you may as well renew the cylinder stat. see list of queries, below.

I don't think there is any relationship between your downstairs rad and present problem.

Queries:

Have you a gas or oil fired boiler, if gas then especially when the cylinder water is cool, it should continue to run (after modulating down) continuously until the cylinder water reaches ~ 50/55C, then it may start cycling on/off until final temperature reached. If oil fired it will cycle on/off continuously so hard to measure except to say the the OFF cycle times will increase as the cylinder temperature rises, it may stay off for 2 or 3 minutes initially and off for 5 to 7 minutes when cylinder getting up to temperature.

Also see where the cylinder stat is located (strapped) to the cylinder, it should be above (maybe by 6" or so) the coil outlet, if below it (unlikely) it won't measure the proper temperature.

Do you live in a hard water area, if so, this will affect the coil transfer and both flow and return will be "roasting", but it takes time for the limescale to build up and if your heating was OK a few days ago then it certainly isn't your problem now.

Thanks for all your help.

Gas boiler.

Cylinder stat in the exact spot that you say.

I suppose this is a new problem. Never happened before. Just can’t make head not tail of it.

John.G

Thanks.

If the boiler (burner) is cutting out when it reaches its set point with a relatively cool cylinder then it is indicative of poor heat transfer, for whatever reason, ie circulating pump problems/setting but if so this would be reflected in slow/poor CH performance so unlikely.

Well if the circulation rate (pump) is OK and the coil inlet/outlet are very hot it really only points to one of two problems, either the coil is fouled (very very unlikely) or there is still air trapped in the coil, is the AAV mounted close to the cylinder coil flow?, if so and emitting no air but the odd drop of water then air unlikely to be your problem.

Re gas boiler, can you give me make/model & power rating.

You said the burner cuts out at 65C, can you time how long it stays off (cycle off time) it should only be for a few minutes before cutting back in again.
Also if CH & HW on together does the boiler run continuously?